Patent GB-242694-A (GB)
Title: Improvements in picture-transmission systems
IPC: H04N1/36
Filed: 1924-08-08
Abstract: 242,694. Western Electric Co., Ltd., (Western Electric Co., Inc.). Aug. 8, 1924. Copying-telegraphs; thermionic generators and modulating systems; filter, rejector, and separator circuits.- In a system of picture telegraphy currents corresponding to the tone values of successive elemental areas of the picture traversed by an exploring device at the transmitting station are arranged to modulate a carrier current of voice frequency which is transmitted over a telephone circuit to a device for reconstituting the picture at the receiving station, and the exploring and reconstituting devices are maintained in synchronism bv the transmission under control of the exploring device of another current of voice frequency. The telephone circuit may be an ordinary line, a phantom circuit, a carrier current channel, or a radio channel. Production of carrier current and its modulation by the picture current. The carrier current is produced by a thermionic valve 140, Fig. 3, with a battery 149 in the grid circuit and with a battery 142 and a choke coil 143 in the plate circuit. The feed back circuit includes a resistance 146 and a transformer 147 with a condenser 148 connected across the transformer primary, and this condenser is adjustable, for determining the frequency in the output circuit which includes a resistance 144 and a potentiometer 145. A stopping condenser 150 prevents current from battery 142 from affecting transformer 147. The output current is impressed on the input circuit of the modulator PM through transformer 153, controlling switches 151, 152, and transformer 133. The circuit ot the photoelectric cell 50, subjected to varying illumination by the device for exploring the picture to be transmitted, includes the leads 58, 59, a source of potential PEB and a resistance 103, and the resulting picture current is amplified by the device PCA before being impressed on the modulator PM. The source PEB includes three adjustable sections 105, 106, 107. The section 107 comprises a potentiometer 110 in series with a battery 111 and a resistance 112. The resistance 103 is in the grid circuit of the first valve 100 of the amplifier PCA, and the plate circuit of this valve includes a battery 114, a resistance 115, and an ammeter 116. The grid circuit oi the second valve 101 includes part of the battery 114 and a biassing potentiometer 118, and the plate circuit includes a battery 120, a resistance 121, and an ammeter 122. The grid circuit of the valve 123 of the modulator PM includes a resistance 134 connected across the secondary of the transformer 133, a biassing potentiometer 124, and a switch 127 enabling the introduction of a batterv 131 for purposes of adjustment. The plate circuit of the valve 123 includes a battery 137, an ammeter 136, and the primary of a transformer 135. The cell 50, leads 58, 59, amplifier PCA, and modulator PM, and their immediate connections are enclosed as shown in an earthed metallic shield 60 supported on rubber pads 139. A potentiometer 155 in the secondary circuit of the transformer 135 is connected through a switch 161 either to the leads 164, 163 of a test circuit, or to output leads 162, 163 connected through a high pass filter SPF to the primary of a transformer 215, Fig. 4. The filter SPF comprises, as shown, three series condensers, two inductive capacity shunts, and an inductive shunt. Production and control of synchronizing current. The synchronizing current is produced by a thermionic valve 170, Fig. 3, with circuit connections similar to those of the valve 140. The output circuit includes a resistance 175 and the primary of a transformer 176 of which the secondarv circuit includes a resistance 182 and a potentiometer 183, and the output current is controlled through leads 39, 62 by the vibration of the fork SF controlling the picture exploring mechanism. Leads 184, 185 connect the potentiometer 183 through a band filter SSF to the primary of the transformer 215. The filter SSF comprises, as shown, two series inductances, two series condensers, and a shunt condenser. Transmission to line, amplifying, and attenuation equalizing. Amplification without distortion is effected at the transmitting station or at the receiving station or intermediately by a twostage amplifier LA, Fig. 4, adapted for handling a ringing current of standard frequency. Resistance 224, condensers 225, 226, relay 227, and the primary of a transformer 217 are associated with the input circuit. The first stage is effected by a valve 216 of which the grid circuit includes battery 239 and the secondary of the transformer 217, and the plate circuit includes battery 235 and choke coil 236. The second stage is effected by a valve 231 of which the grid circuit includes a battery 240 and a potentiometer 237 connected with a condenser 238 across the plate circuit of the valve 216, and the plate circuit includes the battery 235 and choke coil 243. A rheostat 233 and a choke coil 234 are arranged in series with the filament-heating battery 232 for the valves 216 and 231. With the output circuit of the amplifier are associated condensers 228, 229 and 242, relay 230, and, when used at the transmitting station, the primary of a transformer 241 connecting it to the line L. An attenuation equalizer, used at the receiving station, but shown for convenience in the input circuit of the amplifier LA, comprises a condenser 220, an inductance 221, and a resistance 219 connected as shown between the two halves of the secondary of transformer 215, with a resistance 222 and condenser 223 in shunt. Filtering, amplifying, and detecting arrangements at receiving station. The receiving end of the line L is connected in parallel as shown in Fig 5, through filter RPF and potentiometer 301 to the primary of a transformer 302, and through filter RSF and potentiometer 341 to the primary of a transformer 342, the filters RPF and RSF being similar to the filters SPF and SSF, Fig. 4, respectively. The secondary of the, transformer 302 is in the grid circuit of the valve 300 of an amplifier RPA of which the plate circuit includes a batterv 305 and a choke coil 306. The valve 303 of second amplifier RDA is adapted to function alternatively as a detector for use in starting the rotation of the drum carrying the sensitized reproducing surface upon interruption of the carrier current at contact 157 of switch 151, Fig. 3. When the valve 303 is functioning as an amplifier, the grid is polarized by battery 310 of which the circuit is closed at contact 311 of relay 308 and includes a portion only of the resistance 314, and when functioning as a detector the grid is maintained at a larger negative potential by a battery 312 of which the circuit isclosed at contact 313 of relay 308 and includes the whole of the resistance 314. The plate circuit of valve 303 includes batterv 305 and either contact 320 of relay 308 and the primary of a transformer 318, or contact 325 of relay 308, ammeter 330, and relay 309 shunted by condenser 327, according as the valve is to function for amplification or for detection. To convert the device from an amplifier to a detector, a switch 323 is closed to energize relay 308, thus closing at contact 325 the circuit of relay 309. This opens a contact at 350 and closes a holding circuit for relay 308 at contact 324. The magnet 309 is de-energized and the contact at 350 closed to start the transmitting and receiving drums simultaneously by operation of the switch 151, Fig. 3, and the valve 303 thereafter functions as an amplifier. The synchronizing current is amplified by a valve 335 of which the grid circuit includes the secondary of transformer 342, and the plate circuit includes the primary of a transformer 343. The current is then detected by a valve 336 of which the grid circuit includes a battery 345, resistance 344, and the secondary of the transformer 343, and the plate circuit, shunted by condenser 347, includes battery 340, ammeter 351, and leads 348, 349.